Acoustic metamaterials are artificially fabricated materials designed to control, direct, and manipulate sound in the form of sonic, or ultrasonic waves, as these might occur in gases, liquids, and solids. Control of the various forms of sound waves is mostly accomplished through manipulation of the bulk modulus β, and mass density ρ. The density and bulk modulus are analogies of the electromagnetic parameters, permittivity and permeability, respectively, in electromagnetic metamaterials. Related to this is the mechanics of wave propagation in a lattice structure. Also materials have mass and intrinsic degrees of stiffness. Together, these form a dynamic system, and the mechanical (sonic) wave dynamics may be excited by appropriate sonic frequencies (for example pulses at audio frequencies).
Acoustic energy propagation in water depends on two material parameters: the density (approximately 1000 kg/m3) and the bulk modulus (approximately 2.25 Gigapascals) resulting in a fixed speed of sound (approximately 1500 m/s). It is also characterized by its extremely low rigidity, close to zero, which manifests itself in the inability of water to sustain shear waves. The development of a material that could mimic these properties is desirable.